The Finite In 'Fused' Tense Forms

Halliday & Matthiessen (2014: 140):

These ‘fused’ tense forms are in fact the two most common forms of the English verb. When one of these occurs, the Finite did, do(es) will then make its appearance in the subsequent tags and responses, e.g. He gave it away, didn’t he? Yes, he did. But it is already lurking in the verb as a systemic feature ‘past’ or ‘present’, and is explicit in the negative and contrastive forms (e.g. He didn’t give it away; He did give it away).